From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from 69-171-232-181.mail-mxout.facebook.com (69-171-232-181.mail-mxout.facebook.com [69.171.232.181]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E28C81DA57 for ; Tue, 11 Aug 2026 00:09:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=69.171.232.181 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786406995; cv=none; b=TIjXQkhZk7zqi/vRpKgyk7z1Ipve4t2AI9kEAMkcP8LfvSDx1iiG+SLOFAF1groNQyzfC3u00K/4S+wRMLdHaowyKF32TfbskE9FLtp4WWeWT8vAajMeBHx0lt5ieOyDeGY+C2VXlQ13aO6y7d3bQ3oMlD8jz8qdvpjKcIe6lJ8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786406995; c=relaxed/simple; bh=eNudewtbWLgRs65JnKp0RxSdH151dBcKtOJXiN3BMsM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=FZngZK4buoKzAi+jqZxhTokidOiPe8IBw2eTDgCnQ2MMqWaThdJ5817vxe8l+9Bwd7AnWrT6lB6dwZUldzW/DbRQxwD2rF92aq19mtj2OP3QyjmDnlmQYxJEHn1L0AH+Ggu1kfjxGEn1cwUcBEeRCZN4dwMIp9v+kAgptFh1Idg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev; spf=fail smtp.mailfrom=linux.dev; arc=none smtp.client-ip=69.171.232.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=linux.dev Received: by devvm16039.vll0.facebook.com (Postfix, from userid 128203) id 7C9FD233CF26FD; Mon, 10 Aug 2026 17:09:42 -0700 (PDT) From: Yonghong Song To: bpf@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Eduard Zingerman , kernel-team@fb.com Subject: [PATCH bpf-next v4 06/13] bpf: Reject callbacks returning more than 8 bytes Date: Mon, 10 Aug 2026 17:09:42 -0700 Message-ID: <20260811000942.2381774-1-yonghong.song@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260811000911.2378679-1-yonghong.song@linux.dev> References: <20260811000911.2378679-1-yonghong.song@linux.dev> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable A callback handed to a helper or a kfunc (bpf_loop(), bpf_timer_set_callback(), bpf_for_each_map_elem(), ...) is invoked through bpf_callback_t, and an exception callback is invoked by bpf_throw() through u64 (*bpf_exception_cb)(u64 cookie, u64 sp, u64 bp, u64, u64); Both prototypes yield a single u64 in R0, and neither caller has any notion of a second return register, so a callback returning a value in the R0:R2 pair would have the upper half of its return value silently dropped. Reject both at load time: - check_ld_imm(): a callback is materialized as PTR_TO_FUNC by an ld_imm64 pointing at its subprogram, so the subprogram's return convention can be checked where the callback pointer is created, before it ever reaches a helper or kfunc argument. - do_check_common(): an exception callback is not referenced by a PTR_TO_FUNC, it is named by a BTF decl_tag and verified on its own, so check it as its frame is set up, next to the existing "cannot return void" and single-argument checks. Signed-off-by: Yonghong Song --- kernel/bpf/verifier.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index f9a6c3c2132b..0c070957a52b 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -16306,6 +16306,11 @@ static int check_ld_imm(struct bpf_verifier_env = *env, struct bpf_insn *insn) verbose(env, "callback function not static\n"); return -EINVAL; } + if (bpf_ret_reg_pair(env, subprogno)) { + verbose(env, + "callback function with >8-byte return value is not supported\n"); + return -EINVAL; + } =20 dst_reg->type =3D PTR_TO_FUNC; dst_reg->subprogno =3D subprogno; @@ -18528,6 +18533,12 @@ static int do_check_common(struct bpf_verifier_e= nv *env, int subprog) ret =3D -EINVAL; goto out; } + if (bpf_ret_reg_pair(env, subprog)) { + verbose(env, + "exception cb cannot return value larger than 8 bytes\n"); + ret =3D -EINVAL; + goto out; + } =20 /* Also ensure the callback only has a single scalar argument. */ if (sub->arg_cnt !=3D 1 || sub->args[0].arg_type !=3D ARG_ANYTHING) { --=20 2.53.0-Meta